Olivine Tholeiite Research Articles

Two mechanisms for creating large isotopic variations in the upper mantle: Examples from the ronda ultramafic complex

Granulite fades xenoliths recovered from kimberlites in the Colorado-Wyoming State Line and Iron Mountain, Wyoming Districts are primarily two pyroxene-granulite, two pyroxene garnet granulite, and clinopyroxene garnet granulite. An igneous xenolith suite consisting primarily of gabbronorite appears to be gradational with the granulites. No known granulite facies rocks are exposed in this area and the entire nodule population is interpreted as lower crustal in origin.Garnet-clinopyroxene equilibrium temperatures of 580-700°C were obtained for the garnet-bearing granulites using the method of Raheim and Green (1974a). Based on the experimental work of Green and Ringwood (1972), Irving (1974b) and seismic determinations of crustal thickness (50 km) in northern Colorado (Pakiser and Zeitz, 1965), equilibration pressures for the garnet granulite suites are estimated to fall in a range from 10-15 kb.Major element and REE chemistry of analyzed granulite xenoliths show characteristics of continental basalts. Normative compositions of the mafic granulites are mostly equivalent to quartz tholeiite and olivine tholeiite. Relict allotriomorphic granular textures are observed in many samples, and sub-solidus reaction textures present in the garnet-bearing granulites indicate formation of garnet±clinopyroxene at the expense of orthopyroxene, plagioclase, and clinopyroxene.This mafic granulite xenolith suite is believed to represent basic intrusive rocks “equilibrated” under granulite facies conditions at various levels in the lower crust. The mineralogy of the nodule suite suggests that the lower crust in the Colorado-Wyoming State Line region is a predominantly mafic igneous-metamorphic complex.

The mechanics and three‐dimensional internal structure of active magmatic systems: Kilauea Volcano, Hawaii

Interpretation of abundant seismic data suggests that Kilauea's primary conduit within the upper mantle is concentrically zoned to about 34‐km depth. This zoned structure is inferred to contain a central core region of relatively higher permeability, surrounded by numerous dikes that are in intermittent hydraulic communication with each other and with the central core. During periods of relatively high magma transport, the entire cross section of the conduit is utilized. During periods of relatively low to moderate transport, however, only the central core is active. As the conduit penetrates the oceanic crust and enters the volcanic shield, it simultaneously supplies the deeper sections of the rift zones (6‐to 10‐km depth) and the roots of the summit reservoir with picritic magma. The rift zones at depth are inferred to be almost wholly molten and to possess a high degree of fluid continuity from Heiheiahulu in the East Rift Zone, 45 km westward through the roots of the summit magma reservoir, and well into the Southwest Rift Zone. Higher in the shield, the subcaldera magma reservoir and the shallow rift zones occupy the 2‐to 4‐km depth interval. Summit‐differentiated olivine tholeiite (ρ ≈ 2.62 g cm−3) is periodically injected laterally along a horizon of neutral buoyancy within the rift zones, where the density of the magma is just balanced by the in situ density of the shield (Ryan, 1987a, b). Deep rift zone intrusions push seaward the deep tectonic blocks of the volcano's south flank. Shallow rift intrusions build a sheeted dike complex, inferred to be in isostatic equilibrium with the higher‐density deep rift cores below. General finite element analyses are presented for the deformation and stress fields surrounding such dikes in the horizontal and vertical planes. The dike tip in two and three dimensions is surrounded by a tubular core of tensile (σ1, σ2) and shear stress (τmax). The displacement field is characterized by counterrotating cells on either side of the dike tip which, in vertical orientation, produce the characteristic subsidence above the dike complex, with uplift on either side, forming a ridge‐trough‐ridge structure. A finite element model of Kilauea's shield computes the displacement fields and principal stress (σ1) distributions resulting from intrusive activity on each or both of the rift zones. Within the summit region, tensile stress lobes produced by the three‐dimensional upward extension of the intrusions superpose constructively to produce calderawide regimes of tensile stress, conducive to caldera development. Parametric studies of (1) intrusion in the East Rift Zone only, (2) intrusion in the Southwest Rift Zone only, and (3) intrusion in both rift zones demonstrate their unique kinematic contributions. For case 1, the caldera undergoes a counterclockwise rotation (torque up state) conducive to the development of rightstepping en echelon eruptive fissures, as exemplified by the August 14, 1971, eruption. For case 2, the caldera undergoes a clockwise rotation (torque down state) conducive to the development of left‐stepping eruptive fissures, as occurred during the December 31, 1974, eruption. For case 3, the caldera substructure is driven due southward, producing the southward migration of the upper portions of the summit magma reservoir.

Trace element evidence for the origin of ocean island basalts: an example from the Austral Islands (French Polynesia)

The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.

Thorium, strontium and oxygen isotopic geochemistry in recent tholeiites from Iceland: crustal influence on mantle-derived magmas

We present new Sr, Th, and O isotopic results on recent olivine tholeiites and quartz tholeiites from the active rift zones in Iceland. Our data show a negative correlation between 87Sr/ 86Sr ratios and δ 18O values, and a positive correlation between ( 230Th/ 232Th) ratios and δ 18O. These covariations of isotopic ratios strongly support the model proposed by Oskarsson et al. (1982) [1]; which involves a mixing of mantle-derived magmas with crustal silicic magmas produced by melting of old hydrothermally altered rocks under central volcanoes. The hybrid magmas have quartz tholeiite compositions. Thus a large part of the Iceland geochemical anomaly is probably inherited from interaction of the primitive magmas (P-type tholeiites) with the Icelandic crust, and the mantle isotopic heterogeneities in the Icelandic “mantle plume” seem to be rather small.

Structure, composition and age of the Faeroe basalt plateau

Summary The lower Tertiary Faeroe basalt plateau is centred around the Faeroe Islands in the northern part of the supposed Faeroe-Rockall microcontinent. It consists of subaerial lavas, divided into three formations. The lower formation is probably thickest (>3 km) in the southern or central part of the islands, whereas the middle and upper formations are probably thickest (> 2 km in total) just N of the Faeroes, close to the line of opening between the Faeroes and Greenland. The lower formation consists of tholeiites relatively low in TiO 2 . The lower part of the middle formation shows an upward progression from high-Ti olivine tholeiites to high-Ti tholeiites, whereas the entire upper and rest of the middle formations consists of a contrasting population of high-Ti tholeiites and low-Ti MORB-like olivine tholeiites to tholeiites. The MORB type is confined to the northern Faeroes, increasing in abundance upwards. Similar MORB-like basalts have been recovered from further SW along the microcontinent (Faeroe Bank, Bill Bailey Bank, Lousy Bank and the Rockall Plateau), being associated with high-Ti tholeiites near the Faeroes and away from the Tertiary line of opening (Faeroe and Bill Bailey Banks). In addition, transitional to mildly alkaline basalts have been recovered on the E side of the Faeroe Bank Channel and on Bill Bailey Bank. Based on field revision of the magnetostratigraphy of the lower formation it is tentatively suggested that the lower formation was extruded during chrons C26R to C25N and the two higher formations during C24R. Accumulation rates seem to have varied systematically and to have been related to the abundance and type of sediments between the lavas, which may indicate that extrusive activity ceased in the Faeroes before the opening of the NE Atlantic in C24R, and explain the presence of conjugate strike-slip faults in the NW Faeroes. The spatial distribution of the various basalt types fits a model of shifting axes of intraplate volcanicity, shifting from the Faeroes to E Greenland and in part back again.

Early Tertiary volcanic rocks in Well 163/6-1A, Rockall Trough

Summary Well 163/6-1A, located in the northern part of the Rockall Trough, proved the presence of a thick sequence of extrusive igneous rocks below Upper Palaeocene sediments. K-Ar age dating suggests that the lavas were extruded at approximately 55 Ma. The lava sequence comprises three distinct lithologies. The upper part of the pile consists of olivine tholeiites that show alkalic and picritic tendencies and have a distinct within-plate composition. These are underlain by another group of olivine tholeiites that are much closer in composition to normal mid-ocean ridge basalt (N-type MORB). The Sr and Pb isotopic compositions of the basalts suggests possible contamination by continental crustal material. The basalts are underlain by a sequence of cordierite-phyric dacites of remarkably homogeneous composition. Their highly aluminous nature, high Ni and Cr contents and their Sr and Pb isotopic compositions indicate that they are not differentiates of a basaltic parent magma, and are considered to have originated by melting of argillaceous and, possibly, arenaceous rocks at depth. Organic-rich black shale lithologies may have been involved.

Origin of ultramafic xenoliths containing exsolved pyroxenes from Hualalai Volcano, Hawaii

Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K2O in plagioclase and the abundances of TiO2 and Fe2O3in spinel of xenoliths and mid-ocean ridge basalt, and a single 87Sr/ 86Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10−3–1.91×10−3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel.

Anhydrous Partial Melting of Peridotite from 8 to 35 kb and the Petrogenesis of MORB

Equilibrium partial melts under anhydrous conditions have been determined for a MORB pyrolite composition (considered suitable for the production of primary MORB melts) from 8 to 35 kb and a relatively more depleted peridotite composition, Tinaquillo lherzolite, at 15 and 20 kb. The equilibrium partial melts were determined using ‘sandwich’ experiments. The primitive MORB glass DSDP3-18-7-1 was used in experiments using MORB pyrolite while a calculated liquid composition from Jacques & Green (1980) was used in experiments with Tinaquillo lherzolite. The equilibrium partial melts in conjunction with previous studies are used to establish a melting grid from 5 to 35 kb in the CIPW normative basalt tetrahedron. Equilibrium partial melts from MORB pyrolite combined with constraints imposed by the composition of primitive MORB glasses, MORB picrites and primitive trapped glass inclusions are subjected to olivine fractionation calculations to establish the range in composition of primary MORB melts. The results suggest primary MORB melts segregate from MORB source diapirs at pressures of 8 to 25kb in equilibrium with either lherzolite or harzburgite residues. MgO contents of primary MORB melts range from 10–17 wt.% while primary melts with > 17 wt.% MgO are of minor importance. Primary MORB melts range in composition from slightly ne-normative and more hy-normative picrites to olivine and quartz normative olivine tholeiite. Although some primitive MORB glass compositions are possible primary melts from 8–20 kb the majority of primitive MORB glass compositions are derived by olivine fractionation (11–25 wt.%) from more picritic parents from pressures of 15–25kb. The results of this study are in agreement with previous workers suggesting that there is an array of primary MORB melt compositions which evolve to multiple saturation in olivine+ plagioclase + clinopyroxene at different points along a latm. cotectic via differing degrees of olivine fractionation.

Early Tertiary, low-potassium tholeiites from exploration wells on the West Greenland shelf

Data on the Tertiary basalts in the Davis Strait region are reported from two exploration wells drilled by Arco and Mobil on the West Greenland shelf. Hellefisk 1 (67°53 'N, 56°44'W), situated only 60 km east of the mid-line in Davis Strait, penetrated the upper 690 m of a subaeriallava sequence continuous with the onshore volcanics of Disko and situated beneath 2.3 km of Paleocene to Quaternary sediments. The lavas are feldspar microporphyritic tholeiites and mostly unmetamorphosed despite the presence of laumontite and prehnite in the vesicular top zones. Nukik 2 (65°38'N, 54°46'W) penetrated 150 m of hyaloclastites and tholeiitic olivine dolerite sheets, presumably sills, some 200 km further to the south. These vo1canics are also deeply buried and are of unknown extension. The drilled rocks, except for the much altered hyaloclastites in the Nukik 2 well, have low contents of Ti02 (0.99-2.03%), K2O (0.09-0.18%) and P2O5 (0.08-0.21%), La/Sm ratios less than one and 87Sr/86Sr ratios of 0.7032 to 0.7044. Chemically they are related to the MORB-like picrites of Baffin Island rather than the less depleted tholeiites of West Greenland. In both areas the MORB affinity may be related to eruptions through a strongly attenuated lithosphere associated with the opening of Baffin Bay and Davis Strait.

Geochemical evolution of the Juan Fernandez Islands, SE Pacific

The two main islands of the Juan Fernandez group, some 180 km apart are intra-plate volcanoes related to a single hot spot. K-Ar determinations suggest an age difference of c. 3 Ma which is in accord with a spreading rate of c. 6 cm a −1 in this part of the Nazca plate. The younger island of Alexander Selkirk is a relatively recent volcano (<1 Ma) composed of olivine tholeiites, many of them picrites, together with occasional quartz tholeiites and rare trachytes. Robinson Crusoe is an older, deeply dissected island with several separate volcanic centres. The older lavas are predominantly olivine tholeiites, again often picritic, with an occasional quartz tholeiite flow. Alkali basalt flows, never abundant, become more common in the higher part of the succession. Basanite flows are restricted to younger parasitic centres, but basanite dykes and sills cut the older formations. The tholeiites of Alexander Selkirk have a low pressure phenocryst assemblage and their low Ba contents, low Nb/Zr ratios and REE patterns suggest higher degrees of mantle partial melting than their closest counterparts on Robinson Crusoe. The basanites of the latter island have a higher pressure phenocryst assemblage and have higher La/Sm ratios than the tholeiitic lavas. The basanites are not always distinct from the Robinson Crusoe tholeiites on the basis of incompatible trace elements. 86 Sr/ 87 Sr ratios are fairly uniform ( c. 0.7035) in all the rock types but 143 Nd/ 144 Nd ratios are more variable, attaining their lowest values in some of the basanites. Trace element and isotopic data suggest remelting of source regions modified by migration of small degree partial melts over a long period.

Pyroxene oikocrysts in troctolitic cumulates ? evidence for supercooled crystallisation and postcumulus modification

Studies of pyroxene oikocrysts in plagioclase-olivine cumulates crystallised from high alumina olivine tholeiite magmas in layered intrusions inferred to represent periodically replenished magma chambers show that small plagioclases enclosed within the oikocrysts differ significantly from plagioclases in the host rock. Plagioclases within oikocrysts are very elongate with a high surface area, show extensive mutual attachment at high angles resembling skeletal growth, and tend to be randomly orientated. Some are rounded and embayed, are reversely zoned and more sodic by An 5–10 than some of the plagioclase in the host rock. Plagioclase crystal centres tend to be closer spaced in the oikocrysts than in the host rock. In contrast, plagioclases in the host rock are discrete, coarser grained, less elongate, strongly aligned, and locally polygonised.

Tectonic setting of cambrian rifting, volcanism and ophiolite formation in western tasmania

In western Tasmania, small Precambrian regions are surrounded by a ramifying system of troughs filled with Cambrian sedimentary and volcanic rocks, and ophiolite complexes. The volcanic associations include a rift-related olivine tholeiite association, dacite-rhyolite and andesite associations, and a low-Ti, high-Mg andesite-tholeiite ophiolite association, and may have formed during a long-lived period of crustal thinning, punctuated by episodes of crustal rupturing, magmatism, and small scale rifting. Such extensional tectonism could occur in an active continental margin associated with strike-slip faulting of regional scale, and the volcanic associations may together constitute an igneous assemblage characteristic of magmatism in a transcurrent tectonic regime within an active continental margin undergoing break-up. The western Tasmanian Cambrian palaeogeography and volcanism preserve a transitional stage between the Late Proterozoic Kanmantoo regime of sedimentary basins with little volcanism developed at the rifting margin of the Proterozoic craton, and the tectonic regime of the Palaeozoic Lachlan Fold Belt where the Cambrian volcanic rocks are dominated by island-arc associations and the rift-related olivine tholeiite association is absent. Eastern Australian lithosphere may have grown by the insertion of newly-formed igneous complexes within the stretched and rifted continental margin, as well as by the accretion of “terrenes” and the addition of packets of subduction complexes which developed off-shore.

Widespread early Cretaceous flood basalt volcanism in eastern india: Geochemical data from the Rajmahal-Bengal-Sylhet Traps

K-Ar dating of a set of flood basalts from eastern India indicates that volcanism ∼ 115 Ma ago was much more voluminous than heretofore recognised, covering an area of at least 2·10 5 km 2. Separation of India from Antarctica began ∼ 125 Ma ago and in this rift dominated regime, voluminous flood basalts were extruded with the Kerguelen Hotspot perhaps serving as the heat source. We report here on the occurrence of both alkali basalts and olivine tholeiites from the Rajmahal area; preliminary data indicate that more than one type of primary magma was present, one of which resulted from partial melting of a section of the mantle containing metasomatised veins.

Cyclic units in the Somerset Dam layered gabbro intrusion, southeastern Queensland, Australia

The well-preserved Somerset Dam intrusion probably represents a small, relatively shallow, subvolcanic magma chamber. The 500-m-thick exposed sequence consists of 22 macrolayers which are defined by sharp phase, modal and textural contacts. At least six cyclic units, 30–150 m thick, are exposed, and the sequence from the base to the top of a cyclic unit is inferred to be leucogabbro (plagioclase cumulate), troctolite (plagioclase-olivine cumulate), olivine gabbro (plagioclase-augite-olivine cumulate), and oxide gabbro (plagioclase-augite-(olivine)-magnetite-ilmenite cumulate). Mineral compositions in a typical cyclic unit show a reversed fractionation trend in the sequence leucogabbro-troctolite, and a normal fractionation trend from troctolite (the least fractionated rock type) to the oxide gabbro (the most fractionated rock type). The most sensitive parameters for defining the cryptic trends are An in plagioclase, Fo and Ni in olivine, and Cr in magnetite and augite. Whole-rock compositions also show marked changes, and Fe, Ti, V, S and Cu increase and Al, Mg Fe , Cr and Ni decrease from troctolite to oxide gabbro. Despite the remarkable similarity of successive cyclic units, significant differences exist between them in the sequences of layers, thicknesses of individual layers and of the cyclic units, mineral compositions and cryptic patterns, average level of fractionation and the size of the reversals. Unit 3 is particularly unusual because it is the least fractionated and consists of two incomplete subunits. Unit 1, the lowest exposed, is the most fractionated. These differences between the units cannot be explained in terms of a closed system, and are strong evidence for an open system involving periodic injections of magma. The formation of a cyclic unit appears to reflect the dominant control of the order of crystallisation from a batch of replenished magma, which is essentially plagioclase first, followed by olivine, augite, magnetite and ilmenite, and brown hornblende. The parent magma is inferred to have been a high-alumina olivine tholeiite. The gradual rather than sharp reversals in the cryptic trends in the lower part of each cyclic unit may reflect some magma mixing, though postcumulus processes could have been operative.

Sur l'origine oceanique des metabasites de la chaine du mayombe (Gabon, Congo, Zaïre, Angola)

Abstract The Precambrian Mayombian belt is actually considered to be the result of the closure of an oceanic basin located between a western Eburnean craton and an eastern Archaean craton. The cratonic collision, which took place about 1000 Ma ago, induced the tectonic emplacement of ophiolites along the axis of the belt. This collision favoured the formation of the Nyanga's and Niari's epicontinental basins, deformed later during the Panafrican episode. Geochemial studies of the metabasites of the suture confirm their oceanic origin: they are olivine tholeiites similar to abyssal tholeiites. Different from the tholeiites intruded in a lithospheric stretching model, they are almost identical to the tholeiites of the Red Sea. This evolution model for the Mayombian belt: Kibarian narrow ocean floor basins, which closure leads to the Precambrian superior epicontinental basins, is similar to the Burundian of Burundi and to the Kibarian of Zaire. It seems to be general for the post-Eburnean belts in Central Africa.

Origin of spilitic rocks from the southern slope of the Greater Caucasus

A suite of 39 spilitic basalts and andesites of Bajocian age was analyzed for the major elements and Li, Rb, Sr, Ba, Sc, V, Cr, Co, Ni and seven REE, Hf, Ta and Th were analyzed in 20 samples, Sr- and O-isotope ratios have been determined for selected whole-rock samples. The occurrence of biotite and actinolite indicates maximum temperatures of hydrothermal reactions slightly above 400°C. Prehnite, pumpellyite and epidote formed at lower temperatures. Chlorite and sodic plagioclase are abundant products of hydrothermal seawater alteration. A comparison between weakly and intensively altered basalts demonstrates that the largest proportion of Al, V, Cr, Co, Ni and the REE was immobile during spilitization. Enrichment in 18O is correlated with the gain of H 2O, CO 2, Na 2O and Li, which exceeds pre-spilitic levels. Initial 87 Sr 86 Sr ratios for 10 whole-rock samples range from 0.7040 to 0.7049. Relics of fresh clinopyroxene phenocrysts from the spilitic basalts and andesites have initial 87 Sr 86 Sr ratios between 0.7037 and 0.7044 and δ 18O-values between + 4.5 and + 5.6‰ (SMOW). Whole-rock samples have higher 87 Sr 86 Sr init ratios and higher δ 18O-values when compared to their clinopyroxene phenocrysts, which is explained as a result of alteration by seawater. Average δ 18O-values of + 11‰ (SMOW), along with the enrichment of Li, suggest that the major alteration occurred at temperatures below 200°C. The spilitic basalts have higher average Sc, V, Cr, Co and Ni concentrations relative to the spilitic andesites. The abundances of these elements in the spilitic andesites are quite similar to those in typical orogenic andesites. The correlation of Cr and Ni in both the spilitic basalts and the spilitic andesites is explained by various degrees of fractionation of olivine and clinopyroxene. Although Ta and Th may have been mobilized to minor extents, the majority of the analyzed samples has HfTaTh relations which are typical of subduction-related volcanic rocks. Assuming an olivine tholeiite as the parental melt from which the basaltic and andesitic magmas were derived, the average La and Ce concentrations in these rocks require a (metasomatically) enriched peridotite source with 5 times chondritic La and Ce abundances. The coexistence of andesites and basalts at the southern slope of the Greater Caucasus along with the occurrence of ophiolites in the Lesser Caucasus suggests that both could be related to a Middle Jurassic subduction zone in this region.

The volcanic geology and petrology of Isla Pinta, Galapagos archipelago

Pinta, one of the northern islands in the Galapagos archipelago, is the top of a shield volcano which became emergent ∼0.7 m.y. ago. A regional north-north west-trending fracture system influenced the locus of volcanism. Pinta9s geologic development can be divided into an older, shield stage and a younger, fissure stage. Each stage produced a similar, yet distinct, series of evolved olivine tholeiites (magnesium number = 52). The shield series underwent some additional shallow-level fractionation in a periodically replenished chamber. Rare-earth-element data indicate that the mantle source for both of the series has residual garnet. Higher concentrations of incompatible trace elements iri the fissure series imply a lower degree of partial melting. 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios suggest that both series have an enriched, hotspot-type source. Despite close proximity to the Galapagos spreading ridge, Pinta9s lavas lack features characteristic of mid-ocean ridge basalts. The isotopic and rare-earth-element data from Pinta and the other northern islands are consistent with previous models for the Galapagos spreading ridge, which require north-south axially directed transfer of enriched asthenosphere and/or melts from the Galapagos hotspot to the Galapagos Ridge.

Mechanical aspects of postglacial volcanism and tectonics of the Reykjanes Peninsula, southwest Iceland

Four major NE trending postglacial volcanic and tectonic fissure swarms (volcanic systems) occur on the Reykjanes Peninsula, and the westernmost three are the main subject of this paper. Two main types of basaltic volcanoes are associated with these systems: shields of picrite and olivine tholeiite and tholeiite fissures. The average volume of 26 shields is 1.11 km3, and the total production is 29 km3, whereas the corresponding figures for lavas from 101 volcanic fissures are 0.11 km3 and 11 km3. The tectonic fractures are either tension fractures or normal faults of widths up to 20 m, throws up to 10 m, and lengths up to several kilometers. The volcanism and tectonics can be explained by magmatic pressure changes in ellipsoidal magma reservoirs located beneath the fissure swarms. A magmatic pressure increase of the order of 10 MPa is found to be sufficient for an excess uplift of the order of several meters, which is all that is needed to account for the fractures and measured dilation in the fissure swarms. It is concluded that most shield volcanoes, in particular the picrite shields and the large olivine tholeiite shields, formed during the early postglacial period and that their formation was facilitated by the stress field generated as a result of rapid uplift and bending of the crust above the reservoirs. Since that time the reservoirs have become independent systems, the volcanism has been confined to fissures, and the production rate has decreased significantly. During typical fissure eruptions (0.015 km3), only the uppermost several hundred meters of the source reservoir, depending on its magma content, supply magma to the eruption.

Fluorine in basalts from Iceland

The fluorine content of Icelandic tholeiitic and alkaline basalts matches values found in similar rocks from other areas. Covariation between fluorine and incompatible minor elements such as potassium or phosphorus is found in evolved tholeiites and alkali basalts. Lack of such covariation in primitive olivine tholeiites indicates that fluorine and other incompatible minor and trace elements are not controlled by minerals such as amphibole, mica or apatite in the mantle residue, and that the covariation between these elements in the evolved basalts cannot be inherited from the mantle. Model calculations on rocks from the Langjokull area show that olivine tholeiite suites are, if derived by simple fractional crystallization, enriched in incompatible elements much in excess of the increase due to crystal removal. These observations are taken to indicate that the well documented covariation between fluorine and other incompatible elements is not established until evolution of the basaltic magma has started in crustal holding chambers. The constancy of element ratios and enrichment in excess of what can be accounted for by crystal fractionation or incremental addition of new batches of primitive magmas does indicate (1) mineral control involving amphibole, mica or apatite and (2) addition of fluorine, potassium and phosphorous from an external source. It is argued that this source is the crustal envelope of the holding chamber.

Early Tertiary igneous activity west of the Outer Hebrides, Scotland — Evidence from magnetic anomalies and dredged basaltic rocks

Much of the continental shelf off the Outer Hebrides, Northwest Scotland, is underlain by a basement platform which has not undergone the subsidence typical of many parts of the margin west of Britain. Although seismic data suggest that the platform is largely an extension of the Precambrian Lewisian gneiss of the Outer Hebrides the pattern of magnetic anomalies indicates that other lithologies are present. Dredging operations in an area of basement outcrops between St. Kilda and the Flannan Isles have resulted in the recovery of a collection of basaltic rocks in addition to granitic gneisses. Analysis of the freshest basaltic samples reveals that they are olivine tholeiites, typical of “within-plate” basalts, which are chemically distinct from basic intrusions on St. Kilda and other Tertiary volcanic rocks nearby. Whole-rock K-Ar dating yields a mean age of 43.4 ± 1.0 Ma. The association of the basaltic material with one of several prominent linear belts of magnetic anomalies crossing the shelf points to an extensive invasion of the margin by basic igneous intrusions during the Early Tertiary. The dredged basalts appear to represent a late-stage volcanic episode in the evolution of the Tertiary igneous province of Britain. One block of gneiss in the dredge haul has given ages of 404 ± 9 and 431 ± 7 Ma, perhaps indicating reworking of the Precambrian basement by Caledonian tectonic events on the Lewisian foreland. However, further work is necessary to establish the provenance of this gneissic material.